The present invention generally relates to turbine rotors, including those used in steam turbines. More particularly, this invention relates to an alloy suitable for use in high pressure and intermediate pressure stages of a steam turbine rotor and capable of increasing high temperature properties of such a rotor.
Rotors used in steam turbines, gas turbines, gas turbine engines and jet engines experience a range of operating conditions along their axial lengths. The different operating conditions complicate the selection of a suitable rotor material and the manufacturing of the rotor because a material optimized to satisfy one operating condition may not be optimal for meeting another operating condition. For instance, the inlet and exhaust areas of a steam turbine rotor have different material property requirements. High temperature and high pressure conditions within a high pressure (HP) stage at the inlet of a steam turbine typically require a material with high creep rupture strength, though only relatively moderate toughness. On the other hand, a low pressure (LP) stage at the exhaust of a steam turbine does not demand the same level of high temperature creep strength, but suitable materials typically must exhibit very high toughness because of the high loads imposed by long turbine blades used in the exhaust area.
Because a monolithic (monoblock) rotor (i.e., a rotor that is not an assembly) of a single chemistry cannot meet the property requirements of each of the LP, IP and HP stages for the reasons discussed above, rotors constructed by assembling segments of different chemistries are widely used. For example, large steam turbines typically have a bolted construction made up of separate rotor segments contained in separate shells or hoods for use in different sections of the turbine. The steam turbine industry currently favors CrMoV low alloy steels (typically, by weight, about 1% chromium, 1% molybdenum, 0.25% vanadium, up to 0.3% carbon, the balance iron and possibly lesser additions of silicon, manganese, etc. for use in the HP stage and NiCrMoV low alloy steels for use in the LP stage. NiMoV low alloy steels have also been widely used as materials for the various stages. A particular example of a CrMoV alloy contains, by weight, 1.0 to 1.5% chromium, 1.0 to 1.5% molybdenum, 0.2 to 0.3% vanadium, 0.25 to 0.35% carbon, 0.25 to 1.00% manganese, 0.2 to 0.75% nickel, up to 0.30% silicon, the balance iron and incidental impurities, for example, up to 0.010% phosphorous, up to 0.010% sulfur, up to 0.010% tin, up to 0.020% arsenic, and up to 0.015% aluminum.
While rotors fabricated from CrMoV low alloy steel compositions are widely used, the current maximum design temperature for CrMoV steels is about 1050° F. (about 565° C.). As higher inlet temperatures are sought, for example up to about 1065° F. (about 575° C.), to increase steam turbine efficiencies, chromium steel alloys (typically about 9 to 14 weight percent chromium) with varying levels of Mo, V, W, Nb, B must typically be used to meet the higher temperature conditions in the HP stage of the steam turbine. While capable of operating at temperatures exceeding 565° C. within the HP stage of a steam turbine, rotor forgings produced from these alloys incur higher costs and additional measures are often required to address thermal expansion mismatches with alloys used in the cooler stages of the rotor.
Modifications to CrMoV low alloy steels have been made to achieve desired properties for various other applications. For example, CrMoV bolting steels used in steam turbine applications may include additions of aluminum, boron and/or titanium to improve high temperature strength and ductility. Examples include alloys designated as 7 CrMoVTiB 10-10 and 20 CrMoVTiB 4-10. One such bolt alloy composition has been reported to contain, by weight, 0.9 to 1.2% chromium, 0.9 to 1.1% molybdenum, 0.6 to 0.8% vanadium, 0.35 to 0.75% manganese, 0.17 to 0.23% carbon, 0.07 to 0.15% titanium, 0.015 to 0.080% aluminum, 0.001 to 0.010% boron, up to 0.20% nickel, up to 0.40% silicon, up to 0.020% phosphorous, up to 0.020% sulfur, up to 0.020% tin, up to 0.020% arsenic, the balance iron. A particular commercial example is available from Corus Engineering Steels under the name Durehete 1055, and has been reported to contain, by weight, 1% chromium, 1% molybdenum, 0.7% vanadium, 0.5% manganese, 0.25% silicon, 0.2% carbon, 0.1% titanium, 0.04% aluminum, 0.003% boron, the balance iron. Boron has been reported to stabilize V4C3 carbides that serve as a strengthening phase in bolts formed of CrMoV alloys, and titanium has been reported to remove nitrogen from solution to prevent the formation of boron nitride precipitates. However, it is believed that boron has found limited use and titanium has not been used as additives to CrMoV alloys from which rotors are forged. Furthermore, forged steam turbine rotors have vastly different property requirements relative to bolts used in steam turbine applications, for example, to hold two rotor sections together or to hold the two shell halves together for steam containment.